The invention relates to a luminescent material, more in particular to a luminescent material that is suitable for use as a UV-A phosphor in suntanning lamps. The invention relates also relates to a discharge lamp.
An example of a luminescent material that is widely used as UV-A phosphor in suntanning lamps is barium silicate activated with lead (further indicated as BSP). This material combines a high quantum efficiency with a spectral power distribution which is suitable to obtain suntanning lamps with a UV-B content of about 1%. The main drawbacks are a low maintenance and sensitivity towards water. In order to overcome these disadvantages a phosphor particle coating can be applied. However, such a coating is normally applied making use of a wet chemical process and BSP is soluble to a certain extent even at relatively high pH-values. For this reason the coated material exhibits a lower light output than the starting material.
The invention aims to provide a luminescent material that is suitable for use as a UV-A phosphor in a suntanning lamp and has a low solubility in water.
A luminescent material in accordance with the invention includes an alkaline earth borate activated with trivalent cerium of a general formula belonging to the group formed by Ca3-xB2P2O11+0.5x:xCe3+ and Ca2.5-xB5.5P2O15.75+0.5x:xCe3+.
The luminescent materials according to the invention are stable in water, show a high quantum efficiency ( greater than 80%) and can easily be excited with radiation having a wavelength of 254 nm (the wavelength of most the UV radiation generated by mercury in a fluorescent lamp). The maximum of the emission band is situated between 360 and 365 nm. For these reasons the luminescent materials are very suitable to be used in the luminescent screen of a fluorescent suntanning lamp.
In case x divalent calcium ions are replaced by x monovalent ions chosen from the group formed by Na+, K+ and Li+, the resulting luminescent material has no net electrical charge. It has been found that the presence of these monovalent ions improves the quantum efficiency and the absorption of a luminescent material according to the present invention.
Good results have been obtained for luminescent materials according to the invention, wherein 0.025xe2x89xa6xc3x97xe2x89xa60.100.